Troy Rasbury

U-Pb dating of carbonate petrified wood: What these unusual deposits record about conditions of formation

The talk will focus on the discovery of carbonate petrified wood with extremely favorable U/Pb ratios and similar carbonate precipitates found broadly across the Turkana Basin at about 14 Ma. Troy Rasbury will put this in the context of the basin setting at the time based on geodynamic models.

FULL TRANSCRIPT 

I really appreciate being invited, Lawrence and Fred. It's a fantastic meeting. I've learned so much from the history of the discovery. It's just terrific to be invited and to be here. I only met Richard Leakey in 2010 when the Turkana Basin Institute invited a group of geoscientists and people from Marine sciences to go to see their infrastructure at the Turkana Basin, and I got to go to Turkwel and Ileret. And as a geologist, when I got the invitation, I was like, yes, of course I'll go because it's a continental roof system and we love that. And so, I got to meet Richard then he cooked dinner, and he's a very charming person. I couldn't claim that I know him as a friend. And in fact, I think that his legacy is huge. And the thing that I've benefited from, and the thing that I think will really stand the test of time in terms of his legacy is the development of the Turkana Basin Institute. 

And to me, the face of the institute is Lawrence Martin. He's the one that's included me in all the workshops and got me to the Turkana Basin so thank you, Lawrence, I really appreciate that inclusion. And I also appreciate the thought that we're all important as geologists and anthropologist and archeologists and as a team of scientists, we can actually make a big contribution to understanding geology. And certainly, it's inspirational, the work that's been done in the Plio-Pleistocene and is continuing into the Miocene. But to me, the Turkana Basin Institute in its infrastructure, but also just the benefit to Stony Brook and I actually applaud Stony Brook for having the vision to have to contributions too as part of our institution. And it's not just the infrastructure, it's the people. So, one of the things that I'm particularly, that I want to particularly acknowledge is the young faculty that have been brought in through the Turkana Basin Institute, including Greg Henkes and Marine Frouin in my department, but also Tara Smiley, Carrie Mongel, and Natasha Vitek. 

So some of the people that you have or will see speaking here are just some fantastic new scientists that have come to Stony Brook, and they'll also make a contribution to the Turkana Basin but as a Stony Brook advocate, I'm very, I couldn't say thank you enough for, I think, and I do believe that this is going to be a long lived legacy because of this vision of inclusivity and embracing all the different sciences to try to attack a problem. Okay, so I'm talking about uranium/lead dating of carbonate petrified wood, and I'm going sort of march you through a few different pictures of that and talk about sort of the context. But first, I want to give credit to the people that I'm working with, and particularly the Geodynamics group at Stony Brook, including my husband, Bill Holt and Ali Bahadori, and our new colleague, Sam Boone, who I'm very excited to share some early results with you at the end. 

Of course, I'm a geochronologist. I actually call myself a geologist, but I am a geochronologist and I'm a geochemist. And you'll see that with my focus and a lot of the results that I'll present today are from my student, Kevin Hatton, who's just started as an undergrad and he has just been a terrific asset. We were really excited that he decided to stay for his PhD. Katie Wooten, my lab manager. I don't know what I would do without Katie, would be hard. Sidney Hemming, my twin sister and Steven Cox are our colleagues that do Argon geochronology and our other geochronologists friends, Frank Sousa, Elena Steponaitis and Sara Mana. And then one of the really exciting things, and thanks to TBI for bringing Greg Henkes in, is the ability to pair uranium/lead dating of carbonates with temperature. So it's really just sort of a frontier. And we are proud of the fact that we're sort of alone in the world with having those two labs right next to each other. 

And so, the carbonate isotopes, Kevin Hatton has been working with Greg and as student Mae Saslaw has also contributed some of the clump isotope work. But I want to start by saying why I'm excited about the Turkana Basin and Rift in general. This is a figure that shows three, sorry. The top diagram is a three-dimensional model of East African continent or region. And this is a cross section through there. And the main point here is just to say that Bill Holt and Ali Bahadori and their group are actually working to include mantle dynamics along with plate boundary conditions to establish sort of the timing and details of rifting in the Turkana basin. And it starts with sort of early stage rifting where we see maybe just volcanics in the beginning, a little bit of fault formation, but this is a low area that receive sediments. 

And these sediments have the fossils that we're all looking at, and they have petrified wood and they have volcanic rocks that we can date. And so basically it's a hole that accumulates a lot of interesting material that we can think about. And then as it evolves through time, you get maybe different types of volcanic materials and et cetera. So that's the framework from which I'm thinking to me, the trees, the petrified wood are a physical example of the type, what it looked like on the landscape. And certainly there's snapshots, just like the fossils that you guys look at are, but there are very real snapshots. So the size and the types and the density are all important parameters for actually thinking about what the landscape looked like at the time of their formation. So petrified wood, this is a beautiful example from Buluk. You can see that a little tree is deciding to grow inside this sort of trunk by our guards because we are pretty close to the Ethiopian border here in Buluk and littered about the landscape, you can see a lot of these chunks that you see here are actually pieces of petrified wood as well. Lots of petrified wood. 

And so, I gave a practice talk to my daughters, and they said, well, what is petrification? So, here's an explanation for petrification. So we have an event that knocks down some trees, maybe it's a volcanic ash or a lahar or some sort of flood. We get trees that are deposited with sediments and through time, water percolates down through the sediments, picks up ions and as those ions come in contact with the reactive surface that this decaying organic matter is minerals precipitate. And as they precipitate, we call that permineralization and you end up with petrified wood. And in the typical petrified wood, most of us have picked up or bought at a gem and mineral show, is solidified and they're beautiful. But the ones that I'm interested in are carbonate petrified wood. So across the Turkana basin, there's actually a remarkable amount of calcium carbonate and actually dolomite, which I'll talk a little bit more about. 

So, a lot of carbonates and if anybody knows me and some of you do, I date carbonate. So, I like uranium/lead dating of carbonates as my gig. I've been doing it since I was a graduate student and so this is like, oh my gosh, I have something to date here. And of course, if you can date the wood, you're dating something about the things that maybe the animals lived under or on. And so cool, right? As Ellen would say, geo fabulous. Okay, so I'm going to show a few pictures. I think they're beautiful. This is another picture from Buluk showing petrified wood on the surface. These are carbonate petrified wood. So, at Buluk, there's also some solidified petrified wood, but the things that I've focused on have been carbonate because that's what I work on. John Kappelman and Ellen Miller gave me samples from Nakwai and these are beautiful examples of petrified wood. 

And I show these, you can see the branching geometry, you can see the bark, and actually when you cut them open, you can see cellular texture in the, there's a preserved cellular texture. These things must have formed very early after deposition, right? So, the permineralization must be pretty early. In some cases we just see molds of calcium carbonate, and those could have formed many, many years after formation, but it had to be after formation. And then Napudet, is our particular favorite. It's the first place I saw the petrified wood. This is Francis, our field assistant, walking along a long tree trunk here. Sorry, I'm not sure how to do the thing, but you could see there's a tree truck there. And actually a lot of the yellow fragments on the surface are also pieces of petrified wood. This appears to be some sort of a forest that was mowed down by a volcanic eruption so it's a layer, it's a horizon it.

And in some places you can see we're standing around a stump, like the one that we saw at Buluk. And so you can see that there's holes in it, right? And you can see around the trunk there's a halo where geochemical processes have been happening around the root, around this trunk, but you can see is a pretty big trunk. And the minerals that are forming within it are what we're focusing on for dating. And then here I am a bump on the log. I had to show this one, and you can see the logs pretty big here and a bunch of other pieces of petrified wood around it as well. So, getting to the science, Kevin Hatton here, my student is standing next to some logs, and this is his, he actually started working on this project as an undergrad and got some brilliant data and is continuing to work on it. So, we're actually doing the petrified wood but one of the reasons that Napudet is a particularly appealing place to look at it is that the unit that the petrified wood is found in is bracketed by volcanic rocks by basalts. And so, Sydney and Steven have actually dated the basalts, and it's also, also, that's where alesi comes from. 

And so, Kevin has been working on these, this is a, I don’t know why it turned out so yellow, but it's not really a yellow rock, but you can see the bark texture. You can see some sort of interesting diagenesis with the light and the dark parts. And so I took this out to Brookhaven National Lab, and this is an x-ray fluorescence map with a micro probe. And you can actually see the cellular structure in the strontium map of this thing. And so we have other elements that we've mapped as well, but again, that kind of detail gives us the sort of feeling that maybe these things form very quickly after they were deposited. So here, Kevin is in the lab working on say, that rock and standing by his thing there. But the cool thing here is that the basalts that we dated are 14.1 plus or minus 0.2 and 13.5 plus or minus 0.2. 

And then the fossil forest that we've gotten ages from gives an age of 13.5 plus or minus 0.8. Now, this is laser ablation dating of this rock, and we can definitely do better with isotope dilution. We probably should be able to get ages that are on the order of the uncertainty that we get with Argon/Argon. So, the cool thing here is that uranium concentrations are high, uranium/lead ratios are high, and it's a really well-behaved system and it gives an accurate age. And we've tested that by bracketing it with these argon ages so wow. And petrified wood is found across the Turkana Basin through time and space. So we think it's a really exciting avenue for adding additional information to the record. And this is from Napudet, and it's in preparation by Steven Cox at Lamont. Okay, so it's actually dolomite that looks really good. And we found dolomite in a number of places. So, I was just talking about, (pardon me, I'm going, to go backwards again). 

I was just talking about Napudet, which is here. We've also found dolomite in Buluk at Kalodirr west and at Nakwai. So all those samples that actually look the best are dolomite. It's also reported from spring deposits near Kalodirr west there's the 300-meter-high mound that's almost certainly a spring deposit that has dolomite Brocheta in 1992 thesis talks about two-meter-wide walls of dolomite that are actually lining fault. So, there's a lot of evidence for dolomite beyond what we've actually worked on here very exciting. So, dolomitization, that's another thing my daughter said, well, why is dolomite interesting? Well, dolomite is interesting because there's a lot of it in the rock record, but there's not that much in the modern realm. It's hard to go to a place to find how dolomite forms and geologists haven't been able to make it in the lab except by including bacteria. 

So, it is kind of an enigma, and it's called the dolomite problem. So having dolomite sort of across the entire basin is pretty cool. And dolomite with a lot of uranium is pretty cool too. So, it also has strontium and isotopes that look like the mantle. So, the first one that Katie gave me a result on was 0.7033. It's like that's really low. That's mantle-like. So, she's actually done a bunch of strontium and isotopes across all the dolomites that we have in the ranges 0.703 to 0.704. This is very mantle- like it doesn't look like the crust. So it seems like maybe fluids coming up from the mantle. 

It's also very high uranium/lead, and that's not mantle like, but the zirconium/hafnium, the zirconium concentrations are very high, and nobody would've looked at it we just accidentally found zirconium. The zirconium is very high, and the zirconium/hafnium ratios are very high and those are elements that are really hard to separate from each other. The only thing in the world that looks at all like that is carbonatite. So, carbonatite in Russia have very high zirconium/hafnium ratios, and these are higher than that. So, these are the highest, these are higher than anything that's been reported for zirconium/hafnium ratios it’s very unusual. Geochemistry, and again, probably from carbonatite and carbonatites are thought to form at some thousand kilometers in the mantle so pretty far down. Temperatures from Greg's lab, 50 degrees see not that hot, but hotter than surface waters. So I want to turn this upside down. 

Our vision is that instead of being water that's percolating from the top and permineralizing these rocks, we think that fluids are coming from the bottom, interacting with the rocks as they come, picking up that uranium, but creating a petrified wood that's very different than what anybody would've expected. Now you could say nobody's ever looked at it, and if we went everywhere else, maybe we would find similar signals. But I am guessing that's not correct. And so the conclusion is that we are bringing in mantle heat and the fluids and those fluids are involved in the petrification based on the geochemical signature. And the excitement or the importance for us is that we have a way to date these and that they can tell us something about the geodynamics. And so, I'll talk a little bit about the work that Bill and his group are doing with geodynamics to build on that. 

Okay, so before I start the movie, I want to say what I have here. So this is a tomographic map of the, well, actually it shows the mantle dynamic. So, it's basically showing areas that are higher than they should be or lower than they should be based on just the geoid, right? So this is a geophysics map showing the sort of texture, if you will. And so the areas in the warm colors are higher than they would be expected to be in colors that are cool or lower. And you can see that the AA prime here goes across approximately across Turkana. And this section that we have here is from that. So, this is basically a CT scan of the earth from seismic waves. And you can see that there's a temperature difference from a few hundreds of degrees difference in the mantle, which is very hot. So it's few different small percentage of change, but there's a little blob at about a hundred kilometers that's really bright red should draw your attention. And as I play the movie, it's going to move up through the section, it's going to come in contact with that red line, which is the lithosphere asthenosphere boundary.

And it seems like that as it's really approaches that you get some sort of critical, basically reaches some sort of threshold that allows fluids to come up into  the Turkana Basin. And it's really right before rifting, right before significant rifting is when these fluids are actually coming up. And of course a lot of the volcanics are as well. So I'll play the movie. So, you can watch the red blob starting at a thousand kilometers, and that's where people think that carbonatites come from. I might mention that the East African rift has 50% of the world's known carbonatite deposits so that's kind of interesting by itself. And so here's 12 million, you can see that that blob has sort of reached the, or very close to that surface. And we think that that is the source of heat and definitely the geochemical signal that we see in these dolomite deposits. 

And then looking forward, Richard Leakey came to our labs. We were all very thrilled to have the Leakeys, all the Leakeys come to our lab and look at our stuff and tour our labs. And Bill and his former graduate student, Ali Bahadori [00:19:00] did some initial work to try to create landscape models. And this is a picture of Meave and Richard coming over and admiring the work. And Richard told Bill, this is the frontier this is where we need to be going so, I wanted to bring that up. So, I think the geodynamics modeling, and we had a proposal, I had a proposal with Tara actually years ago, and we called it mantled of mammals because we do think that what's happening in the inside the earth is actually driving stuff on the surface of the earth. And if we can tie it all together, it's kind of hard to tie it all together but if you can tie it all together, it's a really cool thing.

So, this here is basically a cross section that's based on the 3D modeling that Ali is doing and the black lines that look like fence posts here are actually spots and each of those spots is a marker. And those markers are actually recording temperature through time. And so, as we reach the surface, that would be like the mineral that thermochronologist would pick up and actually analyze so that would be sort of a test. So, because of the Turkana Basin Institute, I met Sam Boone last year, and he's a thermochronologist from Melbourne University, and we're working together with him now. We've been communicating with him all year. And this is an early result from that looking at the stars or the model that Ali gave to Sam, and the green arrow there is pointing to Turkana. 

And these are comparisons between four different areas that Sam has thermochronology on, and the blue curves are Ali's spots. It's not a perfect fit, it's the first pass, but it's a really exciting avenue to go backwards and forwards in terms of integrating geodynamic models with things that you can actually measure on the ground. So we're very excited about our collaboration with Sam Boone, and I'm just going to say thank you here because I don't have time to bring up this landscape model, but it's really the Turkana Miocene Project, and you'll see a little bit more of this tomorrow from Tara. But Bill Holt, my husband really wanted me to make sure I showed this model. So thank you.

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